The present invention relates to an air-fuel ratio control system for a stratified scavenging two-cycle engine and, more particularly, to a crankcase compression-scavenging method that exhausts the combustion gas by introducing air into the combustion chamber during scavenging and then introduces an air-fuel mixture into the combustion chamber.
In two-cycle engines, as the ignition and explosion of an air-fuel mixture pushes the piston down, an exhaust port opens to begin exhausting the combustion gas. Any remaining combustion gas is exhausted when a scavenging port opens to introduce the air-fuel mixture supplied to the crankcase into the combustion chamber. In certain two-cycle engines, an air passage is connected to the scavenging passage that links the crankcase to the combustion chamber such that the combustion gas is exhausted by first introducing air from the air passage into the combustion chamber when the scavenging port opens and then introducing the air-fuel mixture from the crankcase.
Examples of two-cycle engines that perform stratified scavenging are described in Japanese patent application numbers H9-125966 and H9-287521. These patent applications describe scavenging systems that includes a throttling valve for output control provided in the air-fuel mixture passage and an air valve for flow control provided in the air passage. The throttling and air valves are interlocked by means of a linking mechanism. These engines are designed to eliminate the problem of incomplete combustion and misfiring that are caused by the introduction of a large amount of residual combustion gas into the air-fuel mixture by introducing air into the combustion chamber. These engines are also designed to eliminate the problem of engine performance loss, which is caused by large fluctuations in the flow rate ratio between the air-fuel mixture and the air, by opening and closing the throttling valve and the air valve in an interlocked manner.
Among the machines that use the aforementioned two-cycle engine as the power source, those such as hand-held portable trimmers in particular, are normally operated with the throttling valve opened halfway or fully open from the idling position. However, when they are started, or when an operation is temporarily halted, such as when the operator takes a break or when the machine is moved to another location, the throttling valve, which has returned to the idling position, must be halfway or fully opened again. The throttling valve is opened by the operator pulling a trigger near his hand, and it is closed by the force of the throttling valve return spring. Most operators pull the trigger hard, thereby opening the throttling valve to the halfway or fully open position in a single motion, which abruptly increases the engine revolution speed. This abrupt acceleration tends to occur every time work begins.
When accelerating the engine from the idle revolution region, the fuel supply is increased in correspondence with the rapidly increasing amount of air intake caused by the rapid opening of the throttling valve. The fuel supply is also increased appropriately in the initial stage of acceleration, in which the throttling valve has not opened much, in order to prevent acceleration failure. However, in an engine that prevents the air-fuel mixture from becoming diluted with air by linking the aforementioned throttling valve and the air valve by means of a linking mechanism, the air valve simultaneously begins to open when the throttling valve begins to open from the idling position. Consequently, the problem of acceleration failure, which occurs when the air-fuel mixture required for acceleration becomes diluted, cannot be avoided.
The present invention has been developed in order to solve the aforementioned problem of acceleration performance loss that is inherent in the aforementioned conventional engines that use a linking mechanism for the air-fuel ratio control system. The linking mechanism controls the throttling valve and the air valve in an interlocking manner to maintain the flow rate ratio between the air-fuel mixture and the air to be introduced into the combustion chamber, i.e., the air-fuel ratio, at a nearly constant level. Thus, it would be desirable to provide an air-fuel ratio control system that does not result in acceleration failure, especially acceleration failure that tends to occur in the initial stage of acceleration.
In order to solve the aforementioned problem, the present invention provides, as described below, an air-fuel ratio control system that tends to maintain at a nearly constant level, the flow rate ratio between the air-fuel mixture and the air that is introduced into the combustion chamber of a stratified scavenging two-cycle engine in which an air-fuel mixture passage having a throttling valve for controlling output is connected to the crankcase, and in which an air passage having an air valve for controlling flow rate is connected to the scavenging passage that connects the crankcase to the combustion chamber.
In a first innovative aspect of the present invention, the throttling valve and the air valve may be interlocked by means of gears that are individually coupled to these valves. The gears are designed to not engage with each other when the throttling valve is located between the idling position and the slightly open position, but are designed to engage with each other when the throttling valve is opened beyond the slightly open position, such that the air valve opens and closes in conjunction with the opening and closing of the throttling valve.
In a second innovative aspect of the present invention, the throttling valve and the air valve are interlocked by means of levers that are individually coupled to these valves. The levers are designed to not engage with each other when the throttling valve is located between the idling position and the slightly open position, but are designed to engage with each other when the throttling valve is opened beyond the slightly open position, such that the air valve opens and closes in conjunction with the opening and closing of the throttling valve.
With this design, when the throttling valve is opened rapidly from the idling position, the amount of air does not increase in the initial stage, thereby preventing acceleration failure due to the dilution of the air-fuel mixture. When the throttling valve is opened beyond the slightly open position, the air valve opens and closes in conjunction with the throttling valve, thus eliminating the problems of incomplete combustion and misfiring that are caused by a large amount of residual combustion gas. Engine performance is also maintained by keeping the air-fuel ratio nearly constant, in a manner that engines are intended to function.
In the first innovative aspect of the present invention, which preferably uses gears, the throttling valve rotation angle between the time at which the throttling valve begins to open and the time at which the air valve begins to open, i.e., the delay angle, can be arbitrarily set by adjusting the number of teeth on the driving gear on the throttling valve side and the number of teeth on the driven gear on the air valve side, the number of missing teeth on the gears, or the installation angles of the valve shafts. The change in the opening of the air valve in response to the change in the opening of the throttling valve can also be arbitrarily set by adjusting the speed ratio of the gears, i.e., the gear ratio or the radial ratio of the pitch circles between the gears.
In the second innovative aspect of the present invention, which preferably uses levers, the levers can be installed in any desired manner by adjusting the shape of the driving lever on the throttling valve side and the shape of the driven lever on the air valve side, as well as the installation angles of the valve shafts. The change in the opening of the air valve in response to the change in the opening of the throttling valve can also be arbitrarily set based on the lever ratio.
Note that when the gears are not engaged with each other or when the levers are not engaged with each other, it is desirable to have a return spring bias the air valve in the valve-closing direction in order to keep it fixed in the closed position. It is also desirable to have the throttling valve and the air valve supported by an integrated body in order to prevent errors in gear or lever installation from adversely affecting engine performance.
Other objects and features of the present invention will become apparent from consideration of the following description taken in conjunction with the accompanying drawings.